An optical scanner that is widely used in digital copiers and optical printers uses a collimating lens to convert fight from a light source into a nearly collimated light beam. The nearly-collimated beam is then formed into a line image by an anamorphic lens. A scanner/polarizer that is formed of a rotating polygon mirror, with a polarizing reflective surface on the polygon mirror surfaces, is located near the image-forming position of the line image. The polarized, scanning light beam reflected from the rotating polygon mirror is then collected by an image-forming optical system and the light is imaged onto a target, thereby forming a one-dimensional scan of the target. In such an optical scanner, demand for low cost and miniaturization has become stronger in recent years. Therefore, it is often the case that the collimating lens is formed of a single lens element and that the image-forming optical system is formed of a small number of lens elements, such as two.
One example where there has been an attempt to obtain an optical scanner fit for superfine printing in spite of the optical scanner being compact and formed of a small number of lens elements is the optical scanner described in Japanese Laid Open Patent Publication H10-68903. The scanner disclosed in this publication employs an optical system that is equipped with a diffractive optical element (DOE) that includes a zone plate phase structure. Because a DOE has a property that its relative dispersion is larger (i.e., its reciprocal relative dispersion is smaller) and because the sign of the dispersion is also opposite that of conventional (i.e., refractive) optical systems, DOE's have recently been used in many fields in order to obtain high-precision imaging with fewer lens elements. The optical scanner described in Japanese Laid Open Patent Publication H10-68903 employs a diffractive optical element that includes a zone plate phase structure for its image-forming optical system, as well as an anamorphic lens and collimating lens. Further, it corrects a main-scanning-direction magnification change and focus change due to temperature variation by use of the zone plate diffractive optical element. However, the light source of the optical scanner described in the said Japanese Laid Open Patent Publication H10-68903 is a semiconductor laser, and this type of light source displays a phenomenon known as mode hopping, which makes a high-resolution output difficult to obtain. Thus, it would be desirable to correct the focus error resulting from mode hopping when using a semiconductor laser light source in order to obtain a high-resolution output.
Mode hopping results in the wavelength of the light source shifting from the base wavelength. When there is mode hopping, the refractive index of the light when passing through each lens element changes due to the refractive index being a function of the wavelength. Thus, as the wavelength shifts due to mode hopping, chromatic aberrations result in what is termed a "focus error". This focus error can become a problem even when the chromatic aberration of the collimating lens is itself small, because this chromatic aberration becomes magnified on the scanning target by a ratio, as will now be described.
If we designate F.sub.1 as the focal distance of the collimating lens and F.sub.2 as the focal distance of the image-forming optical system, the chromatic aberration .DELTA..sub.1 (which equals F.sub.1 /.nu..sub.1, where .nu..sub.1 is the reciprocal relative disperson of the collimating lens material at the base wavelength .+-.20 nm) that is generated by the collimating lens is magnified by the factor (F.sub.2 /F.sub.1).sup.2 at the scanning target. In other words, .DELTA.'=.DELTA..sub.1 (F.sub.2 /F.sub.1).sup.2, as set forth in Equation 5, below. For example, using a commonly-used semiconductor laser having a base wavelength of 780 nm, the light source wavelength varies approximately within .+-.20 nm due to mode hopping. To calculate the chromatic aberration .DELTA..sub.1 caused by the collimating lens in this case, given: the focal distance of the collimating lens F.sub.1 is 10 mm, the focal distance of the image-forming optical system F.sub.2 is 210 mm, and the collimating lens material is made of BK-7 glass, which has a reciprocal relative dispersion .nu..sub.1 =612 (at the base wavelength .lambda.=780 nm .+-.20 nm), then .DELTA..sub.1 equals only 0.016 mm. However, the chromatic aberration on the scanning target, .DELTA.', equals 7.2 mm.
Thus, even if the chromatic aberration .DELTA..sub.1 generated at the collimating lens is small, the chromatic aberration .DELTA.' on the scanning target easily becomes too large. Therefore, the lens design needs to make the de-focus tolerance on the scanning target be larger than .DELTA.', and this becomes a difficult design problem if a high-resolution output is required.
With a small number of component lens elements, such as found in conventional arrangements that employ a collimating lens consisting of one lens element and an image-forming optical system consisting of a small number of component lens elements, the focus error due to wavelength shift cannot be prevented, and a high-resolution output has been difficult to attain. Therefore, an optical scanner is desired which can reduce focus errors due to mode hopping and which has a high resolution despite there being only a small number of component lens elements.